As is well known, waveguides can generally be classified as either fundamental mode or overmoded; i.e. the fundamental mode or conventional waveguide is designed with dimensions which only support the fundamental electromagnetic field or mode configuration for propagation in a given frequency band; whereas, the overmoded waveguide is designed so that several modes could be supported, but internal structure is provided to suppress all but the desired modal configuration. As is also well known, conventional waveguide is restricted in maximum power capacity and in minimum loss due to its required cross-sectional dimensions; whereas, overmoded waveguide can be designed to have arbitrarily high power capacity and low attenuation by appropriately increasing the cross-section. As described in the text by A. E. Karbowiak entitled Trunk Waveguide Communication, published by Chapman and Hall, Ltd. (1965), the required suppression of unwanted modes in overmoded waveguide is achieved by using dielectric and metallic structures to restrict allowable modes.
Oftentimes, overmoded waveguide contain a dielectric layer as part of the internal structure for suppression for unwanted modes. For example, in one type of overmoded waveguide used to support the circular TE.sub.01 mode, the wall of the circular waveguide is constructed of an outer conducting pipe with an inner dielectric lining. In another waveguide configuration, an outer conducting pipe has an inner dielectric sheath which in turn supports a helical wound insulated wire. These two types of overmoded waveguide are discussed in the above-noted Karbowiak reference. As a result of relatively low dielectric thermal conductivity, in these types of overmoded waveguide structures, internally generated heat due to propagation loss is not readily dissipated to the outside wall.
Particularly for high power application of such circular overmoded waveguide, the need exists for a simple and effective method and apparatus for cooling the overmoded waveguide. However, this cooling operation must be performed without significantly disturbing the electromagnetic fields, since this would lead to energy loss into unwanted modes.